Liquid crystal displays (LCDs) have been used in not only TV sets with a big screen but also small display devices such as the monitor screen of a cellphone. TN (twisted nematic) mode LCDs, which would often be used in the past, achieved relatively narrow viewing angles, but LCDs of various other modes with wider viewing angles have recently been developed one after another. Examples of those wider viewing angle modes include IPS (in-plane switching) mode and VA (vertical alignment) mode. Among those wide viewing angle modes, the VA mode is adopted in a lot of LCDs because the VA mode would achieve a sufficiently high contrast ratio.
However, in the case of a VA mode LCD, grayscale inversion may occur when the display is viewed from an oblique viewing direction. To prevent such grayscale inversion, an MVA (Multi-domain Vertical Alignment) mode in which multiple liquid crystal domains are formed within a single pixel region has been employed. In an MVA mode LCD, an alignment control structure is provided for at least one of the two substrates, which face each other with a vertical alignment liquid crystal layer interposed between them, so that the alignment control structure contacts with the liquid crystal layer. As the alignment control structure, a linear slit (opening) or a rib (projection) of an electrode may be used, thereby applying alignment control force to the liquid crystal layer from one or both sides thereof. In this manner, multiple (typically four) liquid crystal domains with multiple different alignment directions are defined, thereby attempting to prevent grayscale inversion.
Also known as another kind of VA mode LCD is a CPA (continuous pinwheel alignment) mode LCD. In a normal CPA mode LCD, its pixel electrodes have a highly symmetric shape and either an opening or a projection (which is sometimes called a “rivet”) is arranged on the surface of the counter substrate in contact with the liquid crystal layer so as to be aligned with the center of a liquid crystal domain. When a voltage is applied, an oblique electric field is generated by the counter electrode and the highly symmetric pixel electrode and induces radially tilting alignments of liquid crystal molecules. Also, with a rivet provided, the alignment control force produced on the slope of the rivet stabilizes the tilted alignments of the liquid crystal molecules. As the liquid crystal molecules are radially aligned within a single pixel in this manner, grayscale inversion can be prevented.
Common liquid crystal display devices usually represent colors by additive color mixture of RGB primary colors (i.e., red, green and blue). In general, pixels of a color display panel each include red, green and blue sub-pixels in correspondence with the RGB colors. Such a display is referred to also as a “three primary color display device”. To a display panel of the three primary color display device, YCrCb (YCC) signals which can be converted into RGB signals are input, and based on the YCrCb signals, the luminance values of the red, green and blue sub-pixels are changed. Thus, various colors are represented. In the following description, the luminance value (luminance level) of a sub-pixel corresponding to the minimum gray scale level (for example, gray scale level 0) is represented as “0”, and the luminance value of a sub-pixel corresponding to the maximum gray scale level (for example, gray scale level 255) is represented as “1”. The luminance values of the red, green and blue sub-pixels are each controlled in the range of “0” to “1”.
When the luminance values of all the sub-pixels, i.e., the red, green and blue sub-pixels are “0”, the color displayed by the pixel is black. By contrast, when the luminance values of all the sub-pixels are “1”, the color displayed by the pixel is white. Many of recent TVs allow even a user to adjust the color temperature. In such a TV, the color temperature is adjusted by fine-tuning the luminance value of each sub-pixel. Here, the luminance value of a sub-pixel after the color temperature is adjusted to a desired level is represented as “1”.
Here, change of the luminance of respective subpixels in a common three primary color display device, which occurs when the color displayed by a pixel changes from black to white while it remains achromatic, is described. In an initial state, the color displayed by the pixel is black, and the luminances of the red, green and blue subpixels are “0”. The luminances of the red, green and blue subpixels start to increase. The luminances of the red, green and blue subpixels increase at equal rates. As the luminances of the red, green and blue subpixels increase, the lightness of the color displayed by the pixel increases. When the increasing luminances of the red, green and blue subpixels reach “1”, the color displayed by the pixel is white. In this way, the lightness of the achromatic color can be changed by changing the luminances of the red, green and blue subpixels at equal rates.
However, strictly speaking, when the lightness of an achromatic color is changed, the color displayed by the pixel may sometimes change (see, for example, Patent Document 1). Patent Document 1 discloses performing a gamma correction such that the value of the blue subpixel is higher than those of the red and green subpixels in the process of changing the lightness of an achromatic color. In the liquid crystal display device of Patent Document 1, the sRGB color solid is converted to a color solid of a liquid crystal display panel via a PCS (profile connection space) before a gamma correction is performed with the utilization of a gamma curve in which the value of the blue subpixel is higher than those of the red and green subpixels at middle grayscale levels. Thereby, the change in achromatic color which would occur according to the change of lightness can be prevented. A process of this kind is also called an independent gamma correction process.
In recent years, unlike the above-described three primary color display device, a display device which is designed for additive color mixture of multiple (four or more) primary colors has been proposed (see, for example, Patent Documents 2 to 4). Such a display device which uses four or more primary colors for display is also called a multi-primary color display device. Patent Documents 2 and 3 disclose a multi-primary color display device which has pixels that include red, green, blue, yellow, cyan and magenta subpixels. Patent Document 4 discloses a multi-primary color display device which has another red subpixel in place of a magenta subpixel.
Citation List
Patent Literature
    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-312254    Patent Document 2: Japanese PCT National Phase Laid-Open Publication No. 2004-529396    Patent Document 3: Japanese PCT National Phase Laid-Open Publication No. 2005-523465    Patent Document 4: WO 2007/032133